This invention relates generally to multiple piston pumps and more particularly to a high pressure pump used in a hydraulically actuated electronically controlled unit injector (HEUI) fuel control system. The invention is particularly applicable to and will be described with specific reference to a constant flow, fixed displacement pump and the integration of the fixed displacement pump into a HEUI system. However, those skilled in the art will appreciate that the invention may have broader application and may be integrated into other hydraulic pump driven systems, such as vehicular steering systems.
This invention also relates to a control system for a fixed displacement, constant flow pump and more particularly to a hydraulically actuated electronically controlled unit injector (HEUI) fuel control system using the fixed displacement constant flow pump. The invention is particularly applicable to and will be described with specific reference to a throttling valve controlling metering of low pressure fluid into a high pressure pump used in a HEUI flow control system. However, the invention has broader application and may be applied to other systems using a constant flow, fixed displacement pump requiring fast response over a wide range of operating conditions such as vehicular steering systems as mentioned above.
A) Conventional Systems
As is well known, a hydraulically-actuated electronically-controlled unit injector fuel system has a plurality of injectors, each of which, when actuated, meters a quantity of fuel into a combustion chamber in the cylinder head of the engine. Actuation of each injector is accomplished through valving of high pressure hydraulic fluid within the injector under the control of the vehicle""s microprocessor based electronic control module (ECM).
Generally, sensors on the vehicle impart engine information to the ECM 25 which develops actuator signals controlling a solenoid on the injector and the flow of hydraulic fluid to the injector. The solenoid actuates pressure balanced poppet valves such as shown in U.S. Pat. Nos. 5,191,867 and 5,515,829 (incorporated by reference herein). The poppet valves in the injector port high pressure fluid to an intensifier piston which causes injection of the fuel at very high pressures. The pressure at which the injector injects the fuel is a function of the hydraulic fluid flow supplied the injector by a high pressure pump while the timing of the injector is controlled by the solenoid. Both functions are controlled by the ECM to cause precise pulse metering of the fuel at desired air/fuel ratios to meet emission standards and achieve desired engine performance. Tightening emission standards and a demand for better engine performance have resulted in continued refinement of the control techniques for the injector. Generally the pump flow output has to be variable throughout the operating range of the engine. For example, one manufacturer may desire a constant pump flow throughout an operating engine speed range except at the higher operating engine speeds whereat the injectors are valving so quickly reduced pump flow may be desired even though more fuel is being injected by the injectors to the combustion chambers. Other manufacturers may desire to rapidly change pump flow at any given instant for emission control purposes. For example, the ECM may sense a step load change on the engine and impose a change in the fuel/air ratio to overcome the effects of a transient emission. Still further, the operating vehicular environment severely impacts oil viscosity affecting pump flow and injector performance. Viscosity of the hydraulic fluid is affected by several variables besides heat and is difficult to program into the ECM to fully account for its affect on system performance.
In a HEUI system, high pressure hydraulic actuating fluid is supplied to each injector by a high pressure pump in fluid communication with each injector through a manifold/rail fluid passage arrangement. The high pressure pump is charged by a low pressure pump. As noted in the ""867 patent, the high pressure pump is either a fixed displacement, axial piston pump or alternatively a variable displacement, axial piston pump. If a fixed displacement pump is used, a rail pressure control valve is required to variably control the pressure in the manifold rail by bleeding a portion of the flow from the high pressure pump to a return line connected to the engine""s sump. For example, the ""867 patent mentions varying the output of the high pressure pump by the rail pressure control valve to pressures between 300 to 3,000 psi. A variable displacement pump can eliminate the rail control valve if the flow output of the variable pump can timely meet the response demands imposed by the HEUI system. The pumps under discussion are axial piston pumps in which the pump stroke (displacement) is determined by the angle of the swash plate. Variable displacement, axial piston pumps use various arrangements to change the swash plate angle and thus the piston stroke. Generally speaking, variable output, axial piston pumps do not have the reliability of a fixed displacement, axial piston pump and are more expensive. More significantly, the response time demands for pump output flow in a HEUI system is becoming increasingly quicker and a variable pump may be unable to change output flow within the time constraints of a HEUI system unless a rail pressure control valve is used.
A fixed displacement, high pressure pump is typically used in HEUI systems because of cost considerations. The pump is sized to match the system it is applied to. It is well known that the flow of a fixed displacement pump increases, generally linearly, with speed. Accordingly, the fixed displacement pump is sized to meet HEUI system demands at a minimal engine speed which is less than the normal operating speed ranges of the engine. Higher engine speeds produce excess pump flow which is dumped by the rail pressure control valve to return. The excess flow represents an unnecessary power or parasitic drain on the engine which the engine manufacturers have continuously tried to reduce.
For example, U.S. Pat. No. 5,957,111 shows a control scheme in which excess pump flow is passed to an idle injector but at a rate insufficient to actuate the injector. The system is stated to allow elimination of the rail pressure control valve and permit a more accurate sizing of the fixed displacement pump. However, the system does not avoid unnecessary parasitic engine power drains imposed by the pump. The pump must still be sized to produce a set flow sufficient to actuate the injectors at a low speed and that flow increases with pump speed.
B) The ""167 Patent
The ""167 patent discloses a fixed displacement, axial pump which in contrast to conventional axial piston pumps, eliminates the kidney shaped ports, rotates the cylinder, fixes the swash plate against rotation and establishes an orificed, suction slot inlet for each piston. The suction slot draws a constant volume of fluid into each pump cylinder once pump operating speed is reached to produce a constant flow output from the pump. The pump can therefore be designed to produce the maximum flow required by the HEUI system (i.e., at low operating speeds) which maximum does not increase when pump speed increases as in conventional fixed displacement pumps. The power otherwise expended to drive conventional fixed displacement pumps beyond their designed xe2x80x9cmaximumxe2x80x9d is not required. Improved vehicle performance, better fuel consumption and decreased emissions results because the parasitic power drain is removed.
Additionally, and as noted above, there are times during the vehicle""s operation where less flow from the required xe2x80x9cmaximumxe2x80x9d is sufficient to operate the injectors and desired for better injector performance, enhanced fuel consumption, etc. In the prior applications, it was demonstrated that controlling the flow of fluid to the constant volume high pressure pump by a throttling valve could produce a constant pump output flow at any desired level. The results and benefits achieved by the constant flow pump as discussed above relative to the maximum output sizing consideration, can therefore be achieved throughout the operating range of the pump by a throttling valve at the pump inlet. Parasitic power drains on the system are thus alleviated over the entire operating range of the engine.
The throttling valve generally disclosed in the ""167 patent was simply a solenoid operated valve under the control of the ECM and similar to the high pressure, axial pressure control valve (RPCV) currently used in conventional systems. Because the solenoid valve is controlling the flow of a low pressure pump, its sizing is reduced decreasing its cost. While the solenoid operated valve can throttle the flow to the inlet of the constant flow pump, the viscosity changes in the hydraulic fluid such as the variations that can occur between ambient vehicular start-up temperatures and the sudden fluid flow changes occurring during normal operating conditions, such as that occurring during vehicle acceleration or deceleration, impose requirements on a conventional solenoid valve which are difficult to achieve.
It is therefore a principal object of the invention to provide a fixed displacement multiple piston pump which can be sized for a HEUI or other hydraulic system to alleviate or minimize engine power or parasitic drains imposed on the engine attributed to the associated bleeding of excess capacity pump flow.
This object along with other features of the invention is achieved by a constant flow, fixed displacement, piston pump which includes a non-rotatable cylinder containing a plurality of piston bores spaced about a centerline of the pump. A rotatable shaft having a formed shaft portion is journalled in the pump. Within each bore a piston is movable and has one end extending through a bore end and in contact with the formed shaft portion while the piston""s opposite end is adjacent an outlet check valve at the opposite bore end. The pump has a discharge chamber in fluid communication with all piston outlet check valves and with the pump outlet. Each piston bore has suction slot of set area in fluid communication with the pump inlet which is sized as a function of timed flow through an orifice. The suction slot is transversely positioned at a set distance between the piston bore ends and sealed and opened by axial movement of each piston within its bore whereby fluid displaced into the piston bore decreases during the piston suction stroke in fixed relationship to increases in shaft rotational speed after the operating speed of the pump has been reached to produce a constant displacement pump throughout the operating range of the pump.
An important feature of the invention is achieved by an improvement to an internal combustion engine having a hydraulically actuated, electronically controlled fuel injection system of the type including a fuel injector valving high pressure fluid in response to commands from an ECM to timely inject a metered quantity of fuel to the engine""s combustion chamber. The injector is in fluid communication with the outlet of the high pressure pump which in turn has an inlet in fluid communication with a low pressure pump. The improvement includes a fixed displacement high pressure pump, as described above, which produces a constant output flow of fluid at all operating speeds of the pump whereby the pump can be sized to match the flow demands of a HEUI system without placing excessive or unneeded power demands on the engine.
In accordance with another important aspect of the invention, the improved system includes the provision of a pressure control throttling valve at the inlet of the high pressure pump whereby the generally constant high pressure flow from the high pressure pump can be reduced to lower displacement flow values in response to commands from the ECM without placing any load on the engine to develop a pump pressure higher than what is required to actuate the HEUI system.
In accordance with another aspect of the invention, an annular discharge chamber is in fluid communication with the outlet check valve and the outlet port of the pump. The outlet check valve may be a reed flapper valve whereby high pressure fluid pumped by all cylinders in the pump is united in the discharge chamber to dissipate pump pulsations.
In accordance with a still further aspect of the invention, the high pressure pump has a housing defining a chamber therein and the cylinder is fixed to the housing which also journals the rotatable shaft therein. The housing also has an annular inlet chamber in fluid communication with the bore slots and a drain passage is provided for fluid communication between the housing chamber and the inlet chamber whereby internal pump leakage is drained through the pump inlet avoiding external pump drain lines when the pump operates in a hydraulic system where the pump inlet is not pressurized.
It is an object of the invention to provide a fixed displacement pump having generally constant output flow throughout its operating speeds.
It is a primary object of the invention to provide a fixed displacement pump for use in any vehicular hydraulic system driven by the vehicle""s engine which reduces or minimizes the power drain imposed by the pump on the engine.
It is another object of the invention to provide a fixed displacement pump for use in a HEUI system which provides a constant flow of pressurized fluid over the operating range of the pump to allow a better and/or more consistent control of the injector over the operating range of the engine.
It is another object of the invention to provide a hydraulic circuit for actuating a hydraulically actuated electronically controlled fuel injector which delivers constant pump flow over an operating pump speed range with an ability to throttle the flow on demand while decreasing power demands of the pump on the engine.
Still yet another object of the invention is to provide a fixed displacement pump for use in a HEUI system which alleviates the need for a rail pressure control valve, or, alternatively, allows for use of a smaller, less expensive rail pressure control valve.
Still yet another object of the invention is to provide a fixed displacement pump which is able to provide fluid to a hydraulically actuated, electronically controlled fuel injector that simulates or improves upon the performance level achieved by a variable displacement pump.
Still yet another object of the invention is to provide an improved low cost high pressure pump for use in an HEUI system.
A still further general object of the invention is to provide a fixed displacement pump producing a constant flow of pressurized hydraulic fluid over an operating speed range of the pump for use in any number of vehicular hydraulic systems which use the power from the engine to control the hydraulic system.
These and other objects, features and advantages of the invention will become apparent to those skilled in the art upon reading and understanding the Detailed Description of the Invention set forth below.